The proposed research is a prospective multicenter study to determine if asymptomatic ST depression (ASTD) on ambulatory electrocardiographic recordings (so-called silent myocardial ischemia) is associated with an increased risk of cardiac mortality and morbidity events during a 1 to 3-year follow-up in patients with established coronary heart disease. One thousand patients will be enrolled 2 to 6 months after hospitalization for an acute coronary event. The patients will be derived from geographically dispersed participating centers (10 in the U.S., 1 in Canada, and 1 in Israel) to enhance generalization of the results to the overall population at risk. Ambulatory 24-hour electrocardiographic recordings will be utilized to evaluate the frequency, severity, and duration of ASTD as a marker of silent myocardial ischemia during usual daily activities. This information will be analyzed in conjunction with ST segment depression on treadmill exercise, reversible myocardial perfusion defects on stress thallium testing, and a limited number of prespecified clinical parameters to assess the contribution which ASTD adds to prognostic information ascertained from current diagnostic techniques used to evaluate myocardial ischemia. The main thrusts of the data analyses are: 1) to assess the predictive usefulness and independence of ASTD in ambulatory patients with established coronary heart disease; 2) to evaluate the interactions and associations between the frequency, severity, and duration of ASTD and other measures of myocardial ischemia; 3) to obtain insight into suspected vasoactive (supply) and increased myocardial oxygen consumption (demand) mechanisms of myocardial ischemia; and 4) to better understand the interrelationship between myocardial ischemia and ventricular arrhythmias as it relates to subsequent cardiac mortality and morbidity events.